This invention generally relates to endoprosthesis devices, most often referred to as stents, and more particularly pertains to the radiopaque marking of such devices.
Stents are useful in the treatment of atherosclerotic stenosis in blood vessels and are generally tubular shaped devices which function to hold open a segment of a blood vessel or other anatomical lumen. They are particularly suitable for use in supporting and holding back a dissected arterial lining which could otherwise occlude the fluid passageway therethrough.
In order to accomplish precise placement of stents, various means are employed to identify the position of the stent within a blood vessel. One means frequently described for accomplishing precise placement of a stent is the attachment of small radiopaque markers to the stent so that through the use of fluoroscopy, the position of the stent within a blood vessel can be identified. Once the stent with its radiopaque markers has been implanted, subsequent checkups of the treated segment are easily performed since the markers remain visible under fluoroscopic illumination.
Conventional radiopaque markers, however, have a number of limitations. Upon attachment to a stent, conventional radiopaque markers may project from the surface of the stent, thereby comprising a departure from the ideal profile of the stent. Such conventional radiopaque markers protrude from the walls of the stent and depending upon their location upon the stent, may either project inwardly to disrupt blood flow or outwardly to traumatize the walls of the blood vessel. In addition, conventional radiopaque markers have the disadvantage in that their attachment to the stent can be tedious and imprecise. Moreover, the configuration of many heretofore known markers fails to provide a precise indication of the location and position of the stent. Finally, the galvanic corrosion that might result from the contact of two disparate metals, i.e., the metal used in the construction of the stent and the radiopaque metal of the marker could eventually cause the marker to become separated from the stent which could be problematic should the marker be swept downstream.
Other conventional radiopaque markers restrict the expansion capabilities of an expandable stent by adding rigidity to the stent in areas designated for stent deformation. In other cases, the stents are formed wholly of radiopaque material, such as tantalum, that is highly effective for use in identifying the location of a stent within a vessel, but fluoroscopically illuminates so brightly so as to obscure proper visibility of the blood vessel lesion, thereby impairing the ability to repair the lesion. Finally, conventional radiopaque markers do not generally, under fluoroscopy, provide the operator with means to accurately access the position of the entire length of the stent.
Stents have also been previously marked by plating selected portions thereof with a radiopaque material. An advantageously selected pattern of plated areas would theoretically allow the position, length and diameter of the stent to be discerned. However, due to the minimal thickness of the plating, only an extremely faint fluoroscopic image can be generated which may ultimately limit its utility. Plating may also lead to flaking of the plated material which will embolize and result in undesirable conditions.
To overcome the problems and limitations associated with stents having conventional radiopaque markers, or plated markers, it would be desirable to employ radiopaque markers or markings that do not limit or otherwise interfere in the expansion capabilities of an expandable stent, nor alter the profile of the stent, that are clearly visible, that provide means for visualizing the entire length of the stent, that do not obscure the blood vessel lesion being repaired and that are not detrimentally affected by galvanic corrosion.
The present invention provides for the radiopaque marking of a stent so as to effectively identify the position of such stent, both while fitted to the delivery device as well as upon implantation within a blood vessel, without inordinately obscuring the lesion being repaired. While the marking may form an integral part of a stent, it does not in any way limit the expansion capabilities of the stent. Furthermore, the marking is not adversely affected by galvanic corrosion. The radiopaque marking of the present invention may be adapted to stents having various geometric shapes and that are constructed of any of various materials.
In a preferred embodiment, the stent of the present invention is of wire construction wherein a plurality of shaped wire rings are axially aligned and appropriately linked. This type of stent is well known in the art and offers significant advantages in terms of expandability, radial strength, longitudinal flexibility, and longitudinal stability during expansion. However, no radiopaque materials are known that satisfy the strength and biocompatibility requirements of such application. In order to render such stents fluoroscopically visible in accordance with the present invention, one or more spines of the stent are wholly formed of a radiopaque material. Such construction provides all of the advantages set forth above. Because the spine is wholly constructed of highly radiopaque material, it creates a brilliant image when fluoroscopically illuminated. Its presence along the entire length of the stent serves to unequivocally reveal the position and orientation of the stent. At the same time, the relatively small cross-section of the radiopaque spine serves to minimize the width of the image and therefore only minimally interferes with the visibility of a lesion adjacent thereto.
Additionally, because the spine runs longitudinally along the length of the stent, it does not participate in nor interfere with the deployment of the stent in which all deformation is limited to the radial expansion of the individual rings. Moreover, expansion causes the expanding rings to engage the vessel walls and thus causes the radiopaque spine to become sandwiched therebetween. Should galvanic corrosion cause the spine to become detached from any or all of the expanding rings to which it was originally attached, the spine will continue to be held in place by the radially outwardly directed forces of the expanded rings. Similarly, because each of the rings firmly engages the vessel walls and because the rings may be interconnected by one or more non-galvanically susceptible spines formed of radio transparent material, there is no danger of the rings becoming displaced. Additionally, because the radiopaque spine as well as the radiotransparent spines are located on the exterior of the rings, the flow of blood through the rings is not disrupted thereby and thus platelet activation and thrombosis formation is less likely. This in turn promotes healing and enhances the growth and attachment of endothelium while suppressing the proliferation of growth of smooth muscle cells and excess neointima in the vessels. The minimal thickness of the spines prevents the vessel walls from being traumatized.
The stent may be constructed using any of a variety of techniques including fabricating the series of expanding rings and then linking selected or all of the rings to the radiopaque spine. Any of various methods may be employed to attach the radiopaque spine to the radiotransparent rings.